Add information to "Standard library not found" error

[kLabz]

Added some information to the "Standard library not found" error:

$ unset HAXE_STD_PATH

$ haxe build.hxml                                                    
Standard library not found. You may need to set your `HAXE_STD_PATH` environment variable

$ HAXE_STD_PATH= haxe build.hxml
Standard library not found. Please check your `HAXE_STD_PATH` environment variable (current value: "")

$ HAXE_STD_PATH=/some/invalid/path/:. haxe build.hxml
Standard library not found. Please check your `HAXE_STD_PATH` environment variable (current value: "/some/invalid/path/:.")